Various types of batteries have been developed heretofore, and in every battery, a packaging material is an essential member for sealing battery elements such as an electrode and an electrolyte. Metallic packaging materials have been often used heretofore as battery packages, but in recent years, batteries have been required to be diversified in shape, and desired to be thinner and lighter as performance of, for example, electric cars, hybrid electric cars, personal computers, cameras and mobile phones has been enhanced. However, metallic battery packaging materials that have often been heretofore used have the disadvantage that it is difficult to keep up with diversification in shape, and there is a limit on weight reduction.
Thus, a film-shaped laminate with a base material layer, an adhesive layer, a metal layer and a sealant layer laminated in this order has been proposed as a battery packaging material which is easily processed into diversified shapes and is capable of achieving thickness reduction and weight reduction (see, for example, Patent Document 1). Such a film-shaped battery packaging material is formed in such a manner that a battery element can be sealed by heat-welding the peripheral edge through heat sealing, with the sealant layers facing each other.
Particularly, lithium batteries that are used in notebook-type personal computers and mobile devices (portable devices) such as a smartphone are desired to have a reduced thickness and weight, and battery packaging materials having a very small thickness of about 100 μm are generally used for these batteries. In recent years, further thickness reduction and weight reduction of mobile products, capacity enlargement of lithium batteries, and so on have been desired, and accordingly batteries packaging materials have been required to have a further reduced thickness.
However, when the thickness of a battery packaging material is to be reduced by merely equally thinning a base material layer, a metal foil layer and a sealant layer, the base material layer and the metal layer become excessively thin, leading to an increase in probability of generation of pinholes during molding. On the other hand, reduction of the molding depth for suppressing generation of pinholes causes a serious problem of hindering capacity enlargement of a lithium battery. When the base material layer becomes thin, piercing strength is reduced, leading to bag-breakage of a lithium battery due to impact from the outside, and when the sealant layer is thinned, there arises a problem of impairing a sealing property, an insulation quality and electrolytic solution resistance.